Optical safety beam systems conventionally comprise a paired light emitter and light detector and are commonly used to detect when an object has passed between the two by registering at the light detector when the light beam has been blocked. In general, infrared light is used. A typical application is to prevent a garage door or other mechanism from closing when an object is in the door's path.
With reference to FIGS. 1A-1D, the optical transmitter (TX) 12 typically is installed on one side of a door or area to be monitored. The optical receiver (RX) 14 is installed on the other side. The transmitted light is typically output as a fixed energy level signal that is modulated, such as between 35 kHz and 40 kHz to allow it to be distinguished from steady-state ambient light. If the RX 14 detects the modulated light signal, it reports to a door system 16 that there is no obstruction present between the TX 12 and RX 14 and this indication can be used, e.g., to allow an automatic door to be closed. If the IR beam is obstructed the RX 14 will indicate that the area is blocked and the door control system will not allow the door to close.
The TX 12 and RX 14 are typically placed some distance from each other. To allow for ease of installation and alignment of the RX 12 light detector with the main IR beam 18A from the TX, the emitted light is not fully collimated but instead is allowed to spread so that the detector can be placed anywhere within the light cone. Some of the light 18B from the TX 12 will strike surfaces, such as walls and floors, that fall within the light cone and produce reflected light 18C that reaches the RX 14. If an object moves into the direct light path between the TX 12 and RX 14 but only partially blocks the light cone, the RX 14 may receive this reflected light from the nearby surfaces. Common objects that can partially obstruct a security beam include shopping carts and vehicles with high clearances.
As shown in FIGS. 1A and 1B, if the intervening surface is dull, the reflected light beam 18C will be widely scattered. Any reflected light sent from TX 12 reaching the RX 14 will be weak and fall below the detection threshold of the RX 14. In such a case, a partial obstruction will be detected and the control system 16 prevents the door from closing.
A problem arises when there are reflective surfaces between the TX 12 and RX 14 that fall within the light cone. This scenario is shown in FIGS. 1C and 1D. An example of a typical reflective surface is glazed tile or other polished flooring, or reflective wall surfaces. Also, many dull surfaces may become more reflective when wet or worn down. Because of the high reflectivity, much more of the reflected light will be directed towards the RX. In a partial obstruction situation, the strong reflected IR beam 18D will reach the RX and may be bright enough for the RX to improperly signal that the path is not obstructed. As a result, the control system may allow the door, gate, or other mechanism to close with the potential to cause damage and injury.
Some conventional safety beam systems allow an operator to manually adjust the sensitivity of the detector. A crude compensation for partial obstruction conditions can be achieved in such a system by partially blocking the light beam and adjusting the receiver sensitivity until reflected light is no longer detected. However, this requires manual intervention, making each installation custom. In addition, this technique is not suitable for installations in which the distance the light beam travels between the TX and RX can vary, such as when one of the TX or RX units is mounted on a movable object such as a sliding door or where the TX and RX units are fixed but where the light beam is directed at a reflector mounted on the movable object. As the distance the light beam travels from the TX to the RX decreases, the absolute signal strength received for both the main light beam and the reflected beam increases and vice versa. Similar problems can occur even where the TX/RX distance is fixed if the surface reflectivity along the optical path can vary, such as when it is dry versus being wet or icy.
Another conventional way to avoid this issue is to limit the surface materials present between the TX and RX to things that are dull and do not become reflective. However, many such materials are not aesthetically pleasing. In some cases it may not be practical to use materials with surfaces that remain dull under all expected environmental conditions. A conventional solution is to add a non-reflecting covering. For example, a store may put a black rubber mat in front of an automatic door. In addition to the atheistic impact, this also introduces a potential obstacle in the travel path that may cause tripping.
A need therefore exists for an optical safety beam system that can reliably detect a partial obstruction and that compensate for reflective surfaces that can direct stray light towards the receiver. There is a further need for an automatically adaptive system that can distinguish the main light beam from the reflected beam across a range of potentially varying distances and in response to changes in environmental and other conditions and do so without requiring any manual intervention.
Optical safety beam emitters and receivers are conventionally housed in mechanical assemblies that can be mounted on or within a wall, door jam, or other surface. The assemblies also conventionally include visible LEDs that indicate when the device is operating. Conventional arrangements place the LED relatively far from the photodetector. Some popular configurations use a wide receiver housing where the photodetector and LED are spaced apart on the front face of the receiver. It can be undesirable to have an installation where this large surface is exposed when the unit is installed. Other popular configurations place the LED on a different side from the photodetector. Unfortunately, this side can be blocked when the receiver is installed making it more difficult to quickly determine if the unit is operating. There is a need to provide an improved housing for a receiver that provides a very small visible footprint when installed while also allowing an operating status indicator to be easily seen.